A priest or priestess is an individual able to call upon the power of a Deity in the D-Verse.
The priesthood has always been a significant profession in the D-Verse, with varying manifestations throughout the ages. The first priest was a priest of the Source itself, rising to notability shortly after the separation and isolation of the angels and demons.
The primary purpose of a priest of any kind is to allow the Deities of the Pantheon to directly influence the physical world with their power, without causing the scars on reality that form whenever the aether is disturbed too much. By reverse-engineering the first priest's actions and their consequences, Tieria discovered a loophole in the aetheric framework that governs scars: having a mortal as the focal point of a Deity's intervention, by having them request and/or guide the 'miracle' in question, prevents scars from ever forming in the first place. This limits the extent of the Deities' potential involvement, but avoiding the creation of scars is seen as a more than acceptable trade.
The secondary purpose of a priest is the same as that of any magic user, albeit one unknown to most: to increase the average level of magic in the universe to the point where scars are less likely to form.
The original priests functioned solely as intercessors, requesting the blessings of their various patron Deities and directing them to the best location possible. Because of the low average magic level, the scope of these miracles were extremely limited, the most common form being healing by a priest of the Source. This method also required significant time for meditation and rituals to allow the priests to better focus on the very soft voice of the Deities, and though still in use by the more traditional priests, it has become unnecessary over time.
Roughly around the time Rosewood ascended to the Pantheon, a second form of priesthood emerged, one that used some of the same meditation and rituals but allowed considerably more freedom with the use of a Deity's power. Priests of this second style were able to wield significant power without requesting specific instances of it. Though overuse of the power tended to cause scars, as is often the case, this was curtailed by setting rules for its use - and, if those rules were broken, restriction or even complete withdrawal of the Deity's blessings, leaving the now former priest essentially powerless.
Over time, as the universe's average magic level increased, new and more powerful concepts of priesthood - specifically, magical girls and Shard hybrids - were able to be put into practice without scarring the aether as they would have previously. Both of these concepts are in significantly common use in the Nameless section of the D-Verse.